1. Field of the Invention
The present invention relates to a transmissive liquid crystal display (hereafter called LCD), and particularly to a transmissive LCD having a wide visual angle.
2. Description of the Related Art
As shown in FIG. 1, liquid crystal panel 11 of a conventional normally white type transmissive LCD comprises active matrix substrate 5, color filter substrate 7 formed with a transparent common electrode and a color filter, TN (Twisted Nematic) liquid crystal 6 being impregnated between both substrates 5 and 7, and polarizing plates 4 and 8 which are formed on the outer surfaces of both substrates 5 and 7, respectively, with their axes of polarization inclined 90.degree. to each other. Light emitted from surface illuminant 1 is modulated so as to display a predetermined character or an image. In this TN type LCD, retardation of the incident light varies depending on the incident angle deviating from the normal to liquid crystal panel 11 due to double refraction of the liquid crystal and the quality of its representation greatly depends on the visual angle of the device. On the other hand, a remedy for allowing incident light to fall only in the vertical direction onto liquid crystal panel 11 so as to make retardation constant and diffusing light transmitted through liquid crystal panel 11 has been considered in order to widen the visual angle. Such prior art will hereinafter be described by the following seven examples.
Japanese Patent Laid-Open No. 118518/90 discloses several prior arts: a first prior art in which, as shown in FIG. 2, light applied to LCD cell from light source 101a is made parallel by spherical mirror 101b and lens 101c and the light transmitted through TN type liquid crystal layer 106, is diffused by light diffusion layer 109 made of a transparent body or the like added with scattering particles, and
a second prior art in which, as shown in FIG. 3, light emitted from light source 101 is made parallel by using light waveguide 103 of a honeycomb construction having an high optical density to thereby prevent the reflection of light and the parallelized light is allowed to enter liquid crystal panel 111 with a light diffusion function. However, when spherical mirror 101b and lens 101c are used in the first prior art, the LCD is made thicker. Further, a principal portion of the incident light enters at a substantially right angle to the LCD cell on the entry side of the liquid crystal panel, but diagonally entering light is not fully cut off. In the second prior art, since light waveguide 103 is internally brought to high optical density so as to prevent light from reflection, the diagonal incident light is completely cut off, thereby causing a significant reduction in the quantity of light. Although liquid crystal panel 111 is provided with a light diffusion layer or a color filter having light diffusion function to thereby improve image blur and variation in color tone, a loss of light that goes out from a liquid crystal layer and the reflection of external light take place. According to a third prior art, color filters 120 and 121 formed in two layers are used so as to control or restrain light traveling in the diagonal direction as shown in FIGS. 4(a) and 4(b). Further, according to a fourth prior art, even if a reflected component of external light is reduced by gray filter 114, light emitted from the liquid panel is also reduced simultaneously as shown in FIG. 5. According to a fifth prior art, the reflection of the external light by display pixels cannot be avoided even if black matrix 115 is provided as shown in FIG. 6.
Japanese Patent Laid-Open No. 284731/86 discloses a sixth prior art in which, as shown in FIG. 7, collimator 203 is provided between a light source and plate glass 205 of a liquid crystal panel so that incident light on the liquid crystal panel is substantially parallelized, and the outgoing light modulated by the liquid crystal panel is diffused by microlens 216 corresponding to each pixel to thereby provide a wide visual angle. In the device referred to above, since diagonally traveling light is cut off when the incident light is substantially parallelized, a great reduction in the quantity of light is developed and this device is also affected by the reflection of external light when the light is diffused by microlens 216. Further, a technique is required with which microlens 216 is formed for each pixel.
Further, Japanese Patent Laid-Open No. 299943/87 discloses a seventh prior art in which, as shown in FIG. 8, an illuminating device employed in a transmissive LCD comprising the illuminating device, transmissive liquid crystal panel 311 and lenticular 309 in successive order, is constructed of aperture type fluorescent tube 301a, cylindrical lens 301b, a plurality of semipermeable mirrors 317-1 through 317-4, total reflecting mirror 318 and box 319, and light entering liquid crystal panel 311 is set as quasi-parallel light. In this case, the respective semipermeable mirrors 317-1 through 317-4 are different in reflectance and transmittance to each other and are manufactured by a method of controlling deposition film thickness or a method of forming minute patterns by etching after all of the reflecting mirrors were produced. However, this method is apt to develop stripes at the image.
As described above, the conventional normally white type transmissive LCD is accompanied by a drawback that, since the visual angle is narrow and gradation of the image changes due to variation in retardation as the direction of incident light deviates from the normal to the liquid crystal panel causing a reduction in contrast ratio, an image having a satisfactorily wide visual angle cannot be obtained.
On the other hand, the LCD is made thicker in the aforementioned first prior art and is unsuitable for a notebook-type personal computer. Further, the LCD has no function for reducing the diagonally traveling light deviated from the direction of the normal to the liquid crystal panel. The second prior art has a problem that when the diagonally traveling light is fully cut off using the light waveguide, the quantity of light is greatly reduced. Even if the color filters are used as the two layers so as to suppress the diagonally traveling light as seen in the case of the third prior art, a problem arises that the quantity of light is reduced due to the formation of the color filters as the two layers. Further, no advantageous effect can be brought about in the case of a stripe type color pixel array. Each of the fourth and fifth prior arts is accompanied by a problem that both a loss of light outgoing from the liquid crystal panel and the reflection of external light on the light diffusion side cannot be improved.
Further, the sixth prior art has a problem in that the quantity of light traveling in the diagonal direction is reduced at the collimator and difficulties are encountered in forming the microlenses every pixels.
Furthermore, the seventh prior art involves drawbacks that it is difficult to prevent the stripes from being developed on the image when the semipermeable films different in reflectance and transmittance to each other are formed in the box and also a problem of mechanical strength arises.